Stratified charge engine

ABSTRACT

An internal combustion engine is described comprising a cylinder having an intake port [14], and two manifolds [24, 34] having branches [22, 32] that are configured to supply two gas streams to the intake port [14] of each cylinder. The two streams enter the cylinder separately so as to produce a stratified charge within the engine cylinder. The first manifold [24] supplies a metered quantity of air within which the fuel to be burnt is dispersed and the second manifold [34] supplies dilution gases. A flow obstructing throttle [23] is arranged in each branch [22] of the first manifold to reduce the risk of back-filling of the branches [22] of the first manifold [24].

FIELD OF THE INVENTION

The present invention relates to a stratified charge internal combustionengine fitted with two intake manifolds having branches that areconfigured to supply two gas streams to an intake port of each enginecylinder, the two streams entering each cylinder separately so as toproduce a stratified charge within the cylinder, the first manifoldsupplying a metered quantity of air within which the fuel to be burnt isdispersed and the second manifold supplying dilution gases. Such anengine will hereinafter be referred to as "an engine of the typedescribed initially".

BACKGROUND OF THE INVENTION

Engines are known that comprise two manifolds having branches that areconfigured to supply two gas streams to an intake port of each cylinder,the first manifold supplying a metered quantity of air within which thefuel to be burnt is dispersed and the second manifold supplyingrecirculated exhaust gases (EGR), the EGR gases serving as dilutiongases.

Conventional engines do not achieve charge stratification because thetwo streams are well mixed on entering the cylinder. The EGR gasessupplied through the second manifold during periods that the intakevalve is closed, continue to enter and are stored in the intake port andin the branch of the first manifold. When the intake valve opens again,the intake charge first inducted into the cylinder comprises EGR gasesthat contain little oxygen yet have a high fuel content picked up fromthe walls of the wet intake port. This can be tolerated only if thecombustion chamber is designed to produce good mixing of the chargeduring the compression stroke, as is the case in conventional engines.However, in a stratified charge engine, this storage of EGR gases in thebranches of the first manifold must be avoided as such stratificationwould result in poor combustion quality.

In an engine of the type described initially, it is important to controlthe velocities of the two streams entering the combustion chamber suchthat there is minimum mixing between them during the intake andcompression strokes in order to conserve the stratification up to thetime of ignition. It is for this reason that in the case of swirlingfluid motion in the combustion chamber, the linear velocity of the outerstream in the vortex should be faster than the linear velocity of theinner stream in the vortex such that their angular velocities aresubstantially equal in order to ensure minimum mixing.

In order to produce these velocities, a greater pressure difference mustbe applied along the branch of the faster stream than along the branchof the slower stream. Since the vacuum pressure in the intake port iscommon to both the branches, the vacuum pressure in the plenum chambersof the two manifolds upstream of the respective branches must be setunequally in order to provide the necessary pressure differencesaccordingly.

During the intake stroke of one of the cylinders, the suction in theassociated intake branch would set up unequal pressures in the twoplenum chambers causing balancing flows along the branches of the othercylinders not undergoing suction in the direction to equalise thepressures. These balancing flows are undesirable as the content of onestream would be replaced by the content of the other stream and thedistinction in content between parallel streams would be lost.

WO96/10688 discloses an engine as described initially, wherein anon-return valve is arranged in each branch of the first manifold topermit gas flow in the branch only in the direction towards the intakevalve. This achieves the required objective of maintaining the twostreams separate at all times before they reach the combustion chamberbut has the disadvantage that the non-return valves interfere with theaspiration of the engine under full load operation and reduce themaximum engine power output. To mitigate this effect, the non-returnvalves must be made as large as possible and this in turn leads toproblems of high cost and difficulty in packaging.

SUMMARY OF THE INVENTION

With a view to mitigating the foregoing disadvantages, the presentinvention provides in accordance with a first aspect a method ofoperating an internal combustion engine fitted with two intake manifolds(24,34) having branches (22,32) that are configured to supply two gasstreams to an intake port (14) of each engine cylinder, the two streamsentering each cylinder separately but in parallel with one another so asto swirl about a common axis in the combustion chamber and therebyproduce a charge that is stratified radially from the axis of swirl, themethod comprising the steps of:

supplying by way of the first manifold a first stream of air withinwhich the fuel to be burnt is dispersed,

supplying by way of the second manifold a second stream of dilutiongases, and

maintaining the volume flow ratio and the velocity ratio between the twostreams at the intake port at substantially constant non-zero valuesover a wide range of engine speed and load operating conditions, by

1. maintaining the plenum chambers of the two manifolds at the same loaddependent pressure,

2. partially obstructing each branch of the first manifold whenoperating within said range of engine operating conditions in order toset the desired volume flow ratio between the two streams, the branchesbeing obstructed by means of restrictions of predetermined flow crosssectional area, and

positioning each restriction (23) at a sufficient distance from itsassociated intake port (14) to allow a high velocity jet induced at therestriction to diffuse over the cross sectional area of the firstmanifold branch before the stream reaches the intake port (14), therebyrendering the flow in the branch more uniform and reducing its velocityso as to set the desired velocity ratio between the two streams at theintake port when operating within said range of engine operatingconditions.

According to a second aspect of the invention, there is provided astratified charge internal combustion engine fitted with two intakemanifolds having branches that are configured to supply two gas streamsto an intake port of each engine cylinder, the two streams entering eachcylinder separately but in parallel with one another so as to swirlabout a common axis in the combustion chamber and thereby produce acharge that is stratified radially from the axis of swirl, the firstmanifold supplying a metered quantity of air within which the fuel to beburnt is dispersed and the second manifold supplying dilution gases,wherein means are arranged along the branches of the first manifold forpartially obstructing the flow at times when the engine is operatingwith a stratified charge so as to introduce an additional pressure dropalong the branches of the first manifold in order to render the totalresistance to gas flow along the branches of the first manifoldsubstantially equal to the total resistance to gas flow along thebranches of the second manifold, the flow obstructing means beingpositioned at a sufficient distance upstream of the intake port for theincreased velocity at the flow obstructing means to be dissipated beforereaching the intake port.

The flow obstructing means in the branch supplying the slower streamshould have an obstruction ratio such that for a given volume flow ratiobetween the two streams supplied by the plenum chamber of the twomanifolds, the increased velocity at the flow obstructing means issubstantially equal to the maximum velocity at any point along the otherbranch supplying the faster stream, thus balancing the pressures at thetwo manifolds.

As in WO96/10688, the air/fuel mixture and the dilution gases in thepresent invention are drawn in separately into the engine cylinders,passing over different regions of the intake port and steps are taken tomaintain the two streams of the intake charge separate within thecombustion chamber during the intake and compression strokes of theengine.

However, flow obstructing means in the branches of the first intakemanifold are used in place of non-return valves to prevent back-fillingof these branches with dilution gases. When using non-return valves, thepressures in the two plenum chambers of the two manifolds may differfrom one another and back-filling of the branches of the first manifoldis prevented by the non-return valves. By contrast, in the presentinvention, the flow resistance along the branches of the first andsecond manifolds are balanced in order to render the pressures in theplenum chambers of the two manifolds substantially equal to one another.The absence of a pressure difference between the ends the manifoldbranches that lead to cylinders with closed intake valves eliminates therisk of back-filling of these branches without having to resort toexpensive and bulky non-return valves.

The partial obstruction of the branches of the first manifold isrequired only under low and part load operating conditions, when astratified charge gives advantages of reduced emission of pollutants andreduced fuel consumption. Under high load, it is preferable to revert toa homogeneous charge, whereupon the flow obstruction may be removed sothat it should not interfere with the aspiration of the engine throughthe first manifold.

It is therefore desirable to provide a two-position flow obstructingthrottle in which the closed position affords a preset degree ofobstruction for stratified charge operation and the open positionaffords a minimum of obstruction for homogeneous charge operation. Sucha two-position throttle may conveniently be formed as a butterflythrottle in which the throttle plate is provided with one or more flowapertures.

Instead of placing individual flow obstructing throttles along thelength of the branches of the first manifold, it is possible to obstructthe mouth of the branches at their junction to the common plenum chamberof the first manifold. In this case, a single movable baffle may be usedto obstruct all the branches allowing a simplification of theconstruction and ensuring correctly balanced flow to the differentengine cylinders.

At the flow obstructing throttle the local velocity in the branches ofthe first manifold is intentionally increased to be substantially equalto the flow velocity in the branches of the second manifold at theirintake port ends. However, it is important that the flow velocity in thebranches of the first manifold at their intake port ends should returnto a lower value and this can be achieved by positioning the flowobstructing throttle in the branches of the first manifold sufficientlyfar upstream of the intake ports to allow the increased velocity to bedissipated before the charge reaches the intake port.

If the flow obstructing throttle is formed by a single aperture, thenthe dissipation of the gas velocity will take place over a considerablelength of the manifold branches. For this reason, it is preferred toform the flow obstructing throttle as a plurality of small apertures,for example by forming the plate of a butterfly valve as a wire mesh orgrid. In this case the mean velocity of the gases will rapidly return totheir value upstream of the flow obstructing throttle, therebypermitting the latter throttle to be positioned relatively close to theintake ports.

An engine having the aim of stratifying the charge and having generallythe same components as the present invention except for the obstructingmeans in the branches of the first intake manifold has been proposed inWO95/22687. In the latter patent specification, the first manifold isespecially designed to act as a reservoir for gases supplied through thesecond manifold while the intake valve is closed so that along thelength of the branches of the first manifold there is stored a column ofdilution gases that is stratified along its length and which, if drawninto the cylinder without being disturbed, gives rise to an axiallystratified charge within the engine cylinder.

One of the described embodiments of the latter application ensuresthorough scavenging of the closed end of the intake port during theperiod when the intake valve is closed, that it to say, it makes surethat the intake port is left clean of fuel-air mixture. The reverse flowof dilution gases is then exploited to store within the branch of thefirst intake manifold a stratified charge which is subsequently drawninto the engine cylinder during the intake stroke, still as a column, tocreate axial stratification within the cylinder. The thorough scavengingensures that no isolated pocket of fuel-air mixture is drawn into thecylinder separate from the main mixture charge.

The above application also describes an embodiment which, during theperiod when the intake stroke is taking place, takes advantage of theparallel flows of fuel-air mixture from the first intake duct anddilution gases from the second intake duct to create transversestratification across the engine cylinder. The combined effect in bothembodiments is one of predominately axial charge stratification alongthe cylinder, with stratification across the cylinder existing only as aminor feature.

A requirement of the above system is that the length of the intake ductsmust be sufficient to store the column of stratified charge. Also in thecase where the second intake flow of dilution gases consists of EGRgases, because of the reverse flow into the first intake ducts, theintake port and the first intake duct will alternately contain fuel-airmixture and EGR gases and it is necessary to make sure there is noliquid fuel wetting the walls of the intake port otherwise this wet fuelwill be entrained by the EGR gases which contains no oxygen and cannotsupport combustion. This restricts the application of stratified EGR topre-mixed fuel-air mixture which is already finely atomised or fullyvaporised before it is supplied to the first intake duct which shouldremain dry. This precludes the use of intake port fuel injection whichtypically deposits wet fuel on the intake port walls.

The present invention does not rely on thorough scavenging of the closedend of the intake port during the period when the intake valve isclosed, but avoids the reverse flow of dilution gases into the branchesof the first intake manifold. The branch is not now filled from its endnearer to the intake valve and does not store a column of stratifiedcharge. Instead it remains filled with fuel-air mixture which is drawninto the cylinder as soon as the intake valve is open. This achievesstratification in the combustion chamber by parallel flows and allowsmore freedom in the choice of the length of the intake ducts and thequality of the fuelling system where wall wetting is permissible.

The branches of the second manifold leading to the intake port arepreferably of a substantial diameter as compared with the branches ofthe first manifold, being typically between a quarter and a half of thefull flow cross section of the intake port. It may be considered thatthe presence of such large branches of the second manifold in the intakeports might restrict the breathing of the engine and reduce its fullload capacity when the dilution gas supply to the second manifold isshut off. However, because flow can occur in both directions along thebranches of the second manifold, under conditions when the pressures inthe two manifolds are unequal, the second manifold will act to storegases drawn through the branches of the first manifold while the intakevalves are closed and to transfer gases between branches of the firstintake manifold. As a result, when the supply of dilution gases to thesecond manifold is shut off, both manifolds will be supplyingcombustible mixture to the intake valves.

The dilution gases may either be air, EGR gases or a mixture of the two.In the case of stratification with EGR gases alone, by metering the airsupplied only to the first manifold and setting the fuel quantityaccordingly, it is possible to ensure that the mixture strength withinthe combustible part of the charge is stoichiometric, thereby permittingthe use of a three-way catalyst to purify the exhaust gases.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a four cylinder spark ignition internalcombustion engine fitted with an intake and exhaust manifold systemdesigned to produce a radially stratified charge, and

FIG. 2 schematically shows horizontal and vertical sections through acombustion chamber to show the distribution of fuel and air mixture anddilution gases within the combustion chamber produced by the intakesystem of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 an engine 10 has combustion chambers each with two intakevalves 12 and two intake ports 14, a spark plug 16 and two exhaustvalves 18. Both of the intake ports 14 are supplied with air from afirst manifold that has a plenum chamber 24 and separate branches 22leading to the individual cylinders. The plenum chamber 24 draws inoutside air through a flow meter 52 and a supply throttle 50. Fuel isintroduced into the first manifold near the intake ports by fuelinjectors 60, the fuel quantity being calculated in dependence upon theair flow drawn in through the first manifold only.

A second manifold having a plenum chamber 34 and separate branches 32leading to the intake ports 14 of the cylinders is provided to supplydilution gases under certain operating conditions. To this end theplenum chamber 34 is connected to the exhaust manifold 80 through an EGRpipe 82 and a supply throttle 54 can be used to regulate the flow of EGRgases drawn into the engine. The illustrated embodiment also permits thesecond manifold 34 to be connected to outside air through a divertervalve 90. This valve 90 enables different compositions of the dilutiongases to be used, ranging from 100% EGR to 100% air, or any desiredEGR/air ratio between these two extremes.

As so far described, the engine is generally similar to that describedin WO96/10688. In the present invention however flow obstructingthrottles 23 are provided in the branches 22 to avoid back-filling ofthese branches with dilution gases. These flow obstructing throttles 23introduce an additional pressure drop along the length of the branches22 of the first manifold so as the make the pressure drop over theentire length of these branches substantially equal to the pressure dropacross the length of the branches 32 of the second manifold. In otherwords, the total flow resistance of the branches 22 supplying the slowerstream is increased by the addition of the flow obstructing throttles 23to match the flow resistance along the branches 32 supplying the fasterstream.

The branches 32 of the second manifold at the intake port 14 aredirected tangentially so that as dilution gases enter the combustionchambers, they are directed at a relatively high velocity towards theouter circumference of the cylinder. The branches 22 of the firstmanifold at the intake port 14, on the other hand, are directed towardsthe centre of the cylinder and supply an air and fuel mixture at a lowervelocity. The strong swirl created by the direction and relativevelocity of the two streams produces the charge distribution shown inFIG. 2 in which the shaded region 15 represents the combustible mixtureand the unshaded region 17 represents the dilution gases. Because of thedifference in velocities of the two streams, the combined streams tendto rotate within the combustion chamber as a solid body to maintain thestratification throughout the induction and compression strokes.

The cylindrical shape of the combustible mixture 15 in FIG. 2 assumesthat the two intake valves 12 are opened at the same time and have equallift. By modifying the valve event of one of the valves relative to theother, it is possible to vary the relative sizes of the valve openingsat different times during the intake period. For example, thecombustible mixture 15 may enter through a narrower opening at the startof the intake stroke with the result that the relative proportion ofdilution gases will gradually decrease as the intake stroke progressesresulting in a downwardly facing truncated cone containing thecombustible mixture instead of a regular cylinder. This has theadvantage that for a given total fuel quantity, more of the availablefuel will be concentrated at the top of the cylinder near the spark plugat the instant of ignition to produce a more robust combustion.

In order to achieve a constant degree of charge stratification over arange of engine operating conditions, it is necessary to maintain thevolume of the flows supplied by the first and the second manifolds at asubstantially fixed ratio with one another. This can be achieved bycorrectly sizing the supply throttles 50 and 54 to achieve the desiredvolume flow ratio and ganging the two throttles for simultaneousmovement so as to maintain this ratio constant over a range of throttlepositions. In the invention, a fixed obstruction ratio for the flowobstructing throttle 23 is also selected to match the fixed volume flowratio in the two streams in order to maintain equal pressures in the twoplenum chambers 24, 34 over the stratified charge operating range.

For example in FIG. 1, the effective flow cross sections of the twostreams at the intake port 14 are in the ratio of 1:3. If a 3:1 velocityratio is required between the streams to maintain good stratification,then the supply throttles 54, 50 must be sized in the ratio of 1:1 inorder to supply a 3:1 velocity ratio at the intake port 14. The flowobstructing throttles 23 must be set such that the increased velocity atthe throttles 23 is equal to the maximum velocity in the branch 32 atthe intake port 14 thus incurring the same pressure drop in the branch22 as in the branch 32. Because the flow obstructing throttles 23 arepositioned in the branch 22 sufficiently upstream of the intake port 14,the increased velocity will be dissipated before reaching the intakeport 14 so that the exit velocity ratio of the two streams at the intakeport 14 remains at 3:1 while the pressures in the two plenum chamber arenow balanced.

It should be appreciated from the above example that the volume flowratio through the supply throttles 54, 50, the exit velocity ratio inthe intake port 14 and the obstruction ratio of the flow obstructingthrottles 23 are inter-dependent and must be optimised in relation withone another. Thus it is possible to operate the engine at the samedegree of charge stratification over a wide range of engine load andspeed conditions provided that these ratios are kept constant bysuitable design of each system.

It should be mentioned that long branches 22 are shown in FIG. 1 forclarity. These branches need only be long enough to allow the gases torecover their entry velocity after leaving the flow obstructingthrottles 23. If the throttles 23 are formed as a mesh or grid with manysmall holes, then this recovery occurs in a short distance and the flowobstructing throttles 23 can be positioned close to the intake ports 14.

In FIG. 1, a shut-off valve 56 is positioned in series with the supplythrottle 54 and downstream of the same. When this shut-off valve 56 isopen, then the engine operates with a stratified charge. For homogeneouscharge operation, for example at high engine load, the shut-off valve 56is closed in order to isolate the plenum 34 both from the ambient andfrom the EGR pipe 82. Under this condition, there will be a largepressure difference between the plenums 24 and 34 causing a largebalancing flow along the branches 22 and 32. The second manifold willact to store air and fuel mixture and to transfer the mixture betweencylinders so that the cylinders will receive a combustible mixture fromboth manifolds, albeit that some of the mixture will reach each cylinderindirectly. In this position, the flow obstructing throttles 23 could beallowed to stay in their obstructing position but this would reduce thebreathing of the engine and it is desirable for full load operation toopen the flow obstructing throttles 23 fully. The throttles 23 cantherefore be ganged for operation with the shut-off valve 56 so that theformer is open when the engine is run with a homogeneous mixture and isclosed when the engine is operated with a stratified charge.

Alternatively, the flow obstructing throttles 23 could be coupled to thesupply throttle 50 to open only as the supply throttle 50 is moved nearto the wide open position. To this end the coupling between thethrottles 50 and 23 should include lost motion so that the flowobstructing throttles 23 remain in their obstructing position until fullload operation is approached.

For initial calibration of the obstruction ratio of the flow obstructingthrottles 23, it is possible to use a differential pressure gauge 70connected between the two plenum chambers 24 and 34 and to adjust theobstruction ratio until a zero differential pressure is achieved. Oncethe desired obstruction ratio has been determined, it should not requirevarying or fine tuning as long as the volume flow ratio through thesupply throttles 54, 50 remains the same as during calibration. Inpractice, errors in the ganging of the supply throttles 50 and 54 and inthe partitioned areas of the two streams at the intake port 14 can causevariation from one engine to another and this can be corrected byresorting to a closed loop control system that fine tunes the relativeflows of the two supply streams or varies the obstruction ratio of theflow obstructing throttles 23 to achieve a pressure balance between thetwo plenum chambers 24 and 34.

The illustrated embodiment has multi-point fuel injection. However asthe invention does not critically depend on a dry intake manifold, it isalternatively possible to use a carburettor or a central fuel injectionsystem supplying fuel directly into the plenum 24.

I claim:
 1. A stratified charge internal combustion engine fitted with afirst and second intake manifold having branches that are configured tosupply two gas streams to an intake port of each engine cylinder, thetwo streams entering each cylinder separately but in parallel with oneanother so as to swirl about a common axis in the combustion chamber andthereby produce a charge that is stratified radially from the axis ofswirl, the first manifold supplying a metered quantity of air withinwhich the fuel to be burnt is dispersed and the second manifoldsupplying dilution gases, wherein means are arranged along the branchesof the first manifold for partially obstructing the flow at times whenthe engine is operating with a stratified charge so as to introduce anadditional pressure drop along the branches of the first manifold inorder to render the total resistance to gas flow along the branches ofthe first manifold substantially equal to the total resistance to gasflow along the branches of the second manifold, the flow obstructingmeans being positioned at a distance upstream of the intake port for theincreased velocity at the flow obstructing means to be dissipated beforereaching the intake port.
 2. An internal combustion engine as claimed inclaim 1, wherein the branches of the second manifold at the intake porthave a flow cross sectional area equal to at least one quarter of thetotal area of the intake port of the engine cylinder.
 3. An internalcombustion engine as claimed in claim 2, wherein the branches of thefirst and second intake manifolds at the intake port have approximatelyequal flow cross sections.
 4. An internal combustion engine as claimedin claim 3, wherein the streams of gases from the branches of the firstand second manifolds at the intake port are maintained separate by aphysical partition in the intake port until they reach the vicinity ofthe intake valves.
 5. An internal combustion engine as claimed in claim4, wherein the branches of the first and second manifolds at the intakeport are designed to give substantially parallel flows travelling withdifferent velocities directed tangentially to the cylinder bore of thecombustion chamber so as to produce swirling motion in the combustionchamber about the axis of the cylinder.
 6. An internal combustion engineas claimed in claim 5, wherein fuel is metered in dependence upon theair quantity drawn in through the first intake manifold.
 7. An internalcombustion engine as claimed in claim 6, wherein fuel is metered as aspray into a plenum chamber of the first manifold to mix with themetered air supply.
 8. An internal combustion engine as claimed in claim6, wherein fuel is separately metered as a spray into each branch of thefirst intake manifold downstream of the associated flow obstructingmeans.
 9. An internal combustion engine as claimed in claim 7, whereinthe gases drawn in from the second intake manifold comprise EGR gases.10. An internal combustion engine as claimed in claim 9, wherein theaverage fuel-air ratio of the stratified intake charge isstoichiometric.
 11. An internal combustion engine as claimed in claim 7,wherein the gases drawn in from the second intake manifold contain air.12. An internal combustion engine as claimed in claim 11, whereinrespective supply throttles are provided for regulating the gas suppliesinto the first and second manifolds, the supply throttles being gangedfor movement in unison.
 13. An internal combustion engine as claimed inclaim 12, wherein a diverter valve is located upstream of the supplythrottle leading to the second manifold, the diverter valve supplyingair to the second manifold in a first position, and supplying EGR gasesto the second manifold in a second position.
 14. An internal combustionengine as claimed in claim 13, wherein a shut-off valve is positioned inseries with the supply throttle leading to the second manifold servingwhen closed to isolate the second manifold from ambient air and EGRgases.
 15. An internal combustion engine as claimed in claim 14, whereinthe flow obstructing means comprise a butterfly throttle having aperforated plate to provide a predetermined obstruction to the gas flowin its closed position and movable to an open position in which the gasflow is substantially unobstructed.
 16. An internal combustion engine asclaimed in claim 14, wherein the flow obstructing means comprise abaffle movably mounted in plenum chamber of the first manifold at themouth of the branches of the first manifold.
 17. An internal combustionengine as claimed in claim 15, wherein the butterfly throttle or baffleis coupled for movement with the supply throttles.
 18. An internalcombustion engine as claimed in claim 15, wherein the butterfly throttleor baffle is coupled for movement with the shut-off valve.
 19. Aninternal combustion engine as claimed in claim 18, wherein over a rangeof engine load and speed conditions in which the engine is operated witha stratified charge, the volume of the flows supplied by the first andthe second manifolds are held in a substantially fixed ratio to oneanother and the flow obstructing means have a fixed obstruction ratio.20. An internal combustion engine as claimed in claim 19, wherein eachcylinder has two intake valves having different opening events such thatthe instantaneous ratio of the flow rates of the two streams deliveredto the cylinder varies at different times during the intake period. 21.A method of operating an internal combustion engine fitted with twointake manifolds having branches that are configured to supply two gasstreams to an intake port of each engine cylinder, the two streamsentering each cylinder separately but in parallel with one another so asto swirl about a common axis in the combustion chamber and therebyproduce a charge that is stratified radially from the axis of swirl, themethod comprising the steps of:supplying by way of the first manifold afirst stream of air within which the fuel to be burnt is dispersed,supplying by way of the second manifold a second stream of dilutiongases, and maintaining the volume flow ratio and the velocity ratiobetween the two streams at the intake port at substantially constantnon-zero values over a wide range of engine speed and load operatingconditions, by
 1. maintaining the plenum chambers of the two manifoldsat the same load dependent pressure,2. partially obstructing each branchof the first manifold when operating within said range of engineoperating conditions in order to set the desired volume flow ratiobetween the two streams, the branches being obstructed by means ofrestrictions of predetermined flow cross sectional area, landpositioning each restriction at a sufficient distance from itsassociated intake port to allow a high velocity jet induced at therestriction to diffuse over the cross sectional area of the firstmanifold branch before the stream reaches the intake port (14), therebyrendering the flow in the branch more uniform and reducing its velocityso as to set the desired velocity ratio between the two streams at theintake port when operating within said range of engine operatingconditions.